The present invention relates generally to radiocommunication systems. More particularly, the present invention relates to methods and apparatuses for determining whether to assign an analog or a digital traffic channel to a connection at various decision points, e.g., at call set-up or call handoff.
Conventional cellular mobile radio telephone systems include a land system and many mobile stations. The land system typically includes at least one mobile switching center and a number of radio base stations which are distributed over a geographical area. Each radio base station serves a cell, i.e. a part of the geographical area, by sending and receiving information to and from mobile stations over radio channels. The mobile switching center, connected on one side to the public switching telephone network and on the other to the radio base stations, performs most of the control functions required for calls to and from the mobile stations in the geographical area.
Due to the rapid growth in the number of subscribers requesting radiocommunication service, system demand is rapidly outstripping system capacity, particularly in urban areas. To satisfy this increasing demand for radiocommunication service, system designers have, among other things, implemented technological advances to increase system capacity. For example, while the first radiocommunication systems implemented in the U.S. were analog systems (i.e., systems using analog modulation), more recent systems have been implemented using digital technology to increase system capacity per unit bandwidth.
This evolution in the technology used to provide radiocommunication service has given rise to hybrid mobile stations which are capable of communicating with either analog or digital land systems. Such mobile stations are commonly referred to as "dual-mode" mobile stations. In some geographic areas, digital system service overlaps analog system service, i.e., both analog and digital traffic channels are available to connect mobile stations to a land system to provide radiocommunication service. In such areas, dual-mode mobile stations can either be connected to the land system via an analog traffic channel or a digital traffic channel.
When a mobile station is to be connected to the land system, the land system will identify a traffic channel to handle the connection. A channel can, for example, comprise a pair of frequencies (i.e., in an analog system) or a repeating sequence of timeslots on a pair of frequencies (i.e., in a digital system). This process is typically referred to as channel allocation. For a dual-mode mobile station which is physically located in a geographic area having overlapping analog and digital service, the land system will first decide which type of channel (analog or digital) to allocate to the dual-mode mobile station. Conventionally, this initial selection can be made by defaulting to digital traffic channels. That is, if a mobile station's equipment supports digital communication, then that mobile station will be connected to a digital traffic channel (when available). Alternatively, if the subscriber's equipment only supports analog communication, then that mobile station will be connected to an analog traffic channel. After being allocated to a particular analog or digital traffic channel, the mobile station will be informed of the allocated channel and tune to the corresponding frequencies to receive and transmit radio signals.
Because cells in a radio communication system are typically relatively small, mobile stations often travel between a number of different cells. The process of switching an established connection from a currently serving base station in one cell, to a target base station in another cell, is known as handoff. In existing analog systems, a serving base station determines the need for a handoff of a mobile station whose connection it is handling based on periodic measurements of the signal strength and/or signal quality of the uplink voice channel signals received from that mobile station. If the measured signal strength and/or signal quality is below a predetermined level, the serving base station sends a handoff request to the mobile switching center. The mobile switching center queries neighboring base stations for reports of the previously measured signal strength of signals on the voice channel currently being used by the mobile station. The mobile switching center selects the neighboring base station reporting the strongest signal, provided that the reported signal strength is above a threshold, and transmits appropriate commands to that neighboring base station and to the mobile station via the serving base station to implement the handoff on the same or a different traffic channel.
In dual-mode systems, as specified for example in the EIA/TIA IS-54 standard, handoffs from a digital traffic channel may also be implemented using a Mobile-Assisted Handoff (MAHO) procedure. Using this procedure, a mobile station may be ordered by the land system to measure and report signal strength and/or other parameters of digital radio channels emitted by the serving base station, as well as those emitted by neighbor base stations. This enables handoff decisions made by the land system to be based not only on the measured signal strength and other parameters of the uplink signal received from the mobile station, but also on the downlink signal parameters detected by the mobile station on channels associated with the serving and neighbor base stations. However, when performing a handoff from an analog traffic channel (to either another analog traffic channel or a digital traffic channel), MAHO techniques cannot be used because the mobile station cannot measure signal strength or other signal parameters since the analog traffic channel is continuously monitored, i.e., there is no idle time for measurement.
These conventional techniques for allocating traffic channels at call-setup and call handoff have proven to be problematic for dual-mode mobile stations operating, for example, in geographic areas where digital systems have been migrated or overlaid onto existing analog systems. The different techniques described above to allocate traffic channels in different traffic situations result in widely varying connection reliability. For example, the Applicant has observed that handoffs between analog traffic channels (i.e., analog-to-analog handoffs) have about a 0.5% average connection failure rate, while handoffs from an analog traffic channel to a digital traffic channel have about a 7% average connection failure rate. By way of contrast, handoffs from digital traffic channels (using MAHO) to digital traffic channels have, about a 1% average connection failure rate. Similarly, call set-ups to digital traffic channels have been observed to have about a 2.5% failure rate, while call set-ups to analog traffic channels have only about a 0.7% failure rate.
Considering the above-described differences between connection failure rates for these different traffic cases, it is noted that analog channels can be read simply by tuning to the correct frequency, whereas reading digital channels also requires synchronization to the digital structure of that channel. The increased complexity associated with reading a digital traffic channel can be partially compensated by using MAHO when the handoff is performed from a digital traffic channel. Accordingly, it would be desirable for a system designer and/or a system operator to have the capability to adjust the connection failure rates to suit her or his preferred system design.